Heaters

ABSTRACT

The invention provides a radiant heater comprising a radiative heating element (15, 16); a housing (1), the underside of which is recessed to receive the radiative heating element (15, 16), the radiative heating element being disposed beneath the housing (1) such that its upper half is wholly within the recess, and at least a portion of its lower half protrudes downwardly from the recess; the recess having a heat reflective surface (5a, 5b, 5c, 6a, 6b, 6c) for reflecting heat radiation from the radiative heating element (15,16) in a downwards direction; the housing (1) having means (9) enabling the attachment thereto of a reflective skirt (19) for focusing the radiation emitted from the radiative heating element (15,16).

BACKGROUND OF THE INVENTION

The present invention relates to heaters of the type in which acombustible substance is burnt to release heat. More particularly theinvention relates to radiant heaters for heating industrial buildingssuch as factories, warehouses, hangers and other large structures.

It is known to heat large buildings, and in particular large industrialpremises by means of radiant heaters, and typical radiant heaters usedfor this purpose consist of a U-tube radiator system, a burner such as agas burner being connected to one end of the tube and a fan beingarranged at the other end of the tube for extracting combustion gasesfrom the tube. The U-tube is suspended below a heat reflective housing,which reflects radiation emitted from the tube towards the ground. Sucha heater is disclosed in, for example, British Patent Application GB2145218.

A major problem encountered with such radiant heaters lies in ensuringthat the radiant flux density at ground level is as uniform as possible,and that hot spots and cold spots are avoided. This represents a problembecause whilst a particular form of radiant heater may be configured toprovide optimal heating in a building of one size and shape, it mayprovide a far from ideal heating effect when used in a building of adifferent size and shape. In particular, it has proved difficult tocompensate for variations in the mounting height above ground level, themounting height generally being dependent upon the availability ofsupport structures such as roof support structures on which to mount theheaters.

SUMMARY OF THE INVENTION

The present invention sets out to overcome these problems by providing,in one aspect, a modular heating assembly comprising a basic heater unitto which may be attached a reflective skirt having any one of aplurality of reflector configurations. In accordance with one aspect ofthe invention, a reflective skirt of a particular configuration may beselected to provide the desired reflective pattern and consequentradiant flux density at a given location at ground level in a building.

A further object of the invention is to improve the radiant efficiencyof such heaters, and to minimize heat losses due to conduction andconvection.

A still further object of the present invention is to provide a heaterwhich is more efficient, in the sense that a greater heating effect isobtainable for the same consumption of fuel, or alternatively that lessfuel is required to achieve the same heating effect.

Accordingly, in a first aspect, the invention provides a process forfitting a heating system in a building, the heating system comprisingoverhead radiant heaters with skirt portions for directing the radiatedheat towards the floor of the building, the process comprising the stepsof:

(i) measuring the building floor area A;

(ii) determining the desired temperature rise ΔT above ambient;

(iii) from A and ΔT, determining the required floor radiant flux densityQ;

(iv) from Q and the performance of a heater, determining the number N ofheaters;

(v) from A and N, determining a desired floor radiant flux pattern foran individual heater;

(vi) from the floor radiant flux pattern, selecting a skirt having aconfiguration to achieve that pattern;

(vii) attaching the selected skirt to a heater; and

(viii) installing the heater and skirt in the building.

In a second aspect, the invention provides a radiant heater comprising aradiative heating element; a housing, the underside of which is recessedto receive the radiative heating element, the radiative heating elementbeing disposed beneath the housing such that its upper half is whollywithin the recess, and at least a portion of its lower half protrudesdownwardly from the recess; the recess having a heat reflective surfacefor reflecting heat radiation from the radiative heating element in adownwards direction; the housing having means enabling the attachmentthereto of a reflective skirt for focusing the radiation emitted fromthe radiative heating element.

The radiative heating element may be, for example, a radiant heater tubeheated by a gas burner, or may be an electrically heated heatingelement.

In one embodiment, the radiant heater comprises a tube, a burnercommunicating with one end of the tube; and extraction meanscommunicating with the other end of the tube for extracting combustiongases from the tube.

The recess on the underside of the housing is preferably in the form ofa channel, e.g. an elongate channel. Thus the radiative heating elementis disposed beneath the housing such that its upper half is whollywithin the channel, and at least a portion of its lower half protrudesdownwardly from the channel.

In one particular embodiment, there is provided a radiant heatercomprising a tube, a burner communicating with one end of the tube;extraction means communicating with the other end of the tube forextracting combustion gases from the tube; a housing, the underside ofwhich defines a channel; the tube being disposed beneath the housingsuch that its upper half is wholly within the channel, and at least aportion of its lower half protrudes downwardly from the channel; thechannel having a heat reflective surface for reflecting heat radiationfrom the tube in a downwards direction; the housing having means forenabling the attachment thereto of a reflective skirt for focusing theheat radiation.

Preferably the housing is provided with a plurality of mounting bracketsfor attachment beneath the housing at spaced intervals thereon; themounting brackets having first mounting means for supporting the tubebeneath the housing, and second mounting means for the attachmentthereto of the reflective skirt.

The housing may carry a plurality of radiant heater tubes. Preferablythe radiant outputs of the tubes are matched such that the radiantheater has a substantially uniform total radiant output along itslength.

In one embodiment, the channel is divided into a pair of sub-channelsarranged side by side and separated by a central barrier, eachsub-channel having a tube disposed therein. Preferably the two tubesconstitute the two limbs of a U-tube burner, the burner being arrangedto communicate with one end of the U-tube, and the extraction meansbeing arranged to communicate with the other end.

Preferably, the central barrier tapers in a downwards direction. Mostpreferably the central barrier extends downwardly such that its loweredge is aligned with lower edges of the walls of the channels.

The central barrier typically has opposing walls which have an angle ofinclination or a curvature which mirrors an angle of inclination orcurvature of an opposing wall of the channel.

Preferably the channel and/or each sub-channel functions in the mannerof a parabolic reflector.

For ease of construction, and to enable the housing to be fabricated bysimple metal bending operations, the channel and/or each sub-channel mayhave a generally planar upper surface, and generally planar sidesurfaces diverging downwardly from the edges thereof.

The mounting bracket may be attached to the housing by any suitablemeans, and for example may be bolted, riveted, or welded so as toprovide a removable or fixed connection with the housing.

The housing may advantageously be thermally insulated to limit heat lossthrough conduction and convection via the upper and side surfacesthereof.

Accordingly, in a third aspect, the invention provides a radiant heatercomprising: a radiative heating element; a housing, the underside ofwhich is recessed to receive the radiative heating element, theradiative heating element being disposed beneath the housing such thatits upper half is wholly within the recess, and at least a portion ofits lower half protrudes downwardly from the recess; the recess having aheat reflective surface for reflecting heat radiation from the radiativeheating element in a downwards direction; wherein at least a portion ofthe housing surrounding the recess is thermally insulated to reduce heatloss through the housing.

The radiative heating element may take the form of a radiant heater tubeheated by a gas burner or an electrically heated heating element ashereinbefore defined. For example the heating element may comprise: atube, a burner communicating with one end of the tube; and extractionmeans communicating with the other end of the tube for extractingcombustion gases from the tube.

The housing can have a downwardly open channel on the underside thereof,within which is mounted the tube; the channel having a heat reflectivesurface for reflecting heat radiation from the tube in a downwardsdirection; and wherein at least a portion of the walls defining thechannel have on or adjacent an upper surface thereof a layer of thermalinsulation material.

In one embodiment, the housing comprises inner and outer skins, theinner skin defining the walls of the channel and the outer skin definingthe upper surface of the housing, the space between the inner and outerskins being at least partially filled with thermal insulating material.

The thermal insulating material is preferably one which is capable ofresisting temperatures in excess of 500° C., and in particulartemperatures above 600° C.

In order to improve the reflective efficiency of the reflective surfacesof the channel, the reflective surfaces are preferably surfaces whichhave been treated to reduce surface porosity and unevenness and improvereflectance. For example, the surfaces may be of anodised aluminium, andin particular may be formed of a coloured anodised aluminium, mostpreferably a gold coloured anodised aluminium. Gold coloured anodisedaluminium is considered to be particularly efficient at reflectingradiation in the context of the heaters of the present invention.

In order to allow the angle of spread of the radiation emitted from theheater to be adjusted, the heater will preferably be provided with meansfor adjusting the height of the tube within the housing. For example,the means for adjusting the height of the tube within the housing maytake the form of adjustable length cables extending between opposingmounting points on the mounting bracket, on which cables, the tube ortubes is or are supported, the cables being adjustable such thatshortening the cable results in the raising of the tube, whilstlengthening the cable results in the lowering of the tube.

The cable may take the form of a flexible stainless steel cable havingnon-ferrous mountings and length adjusters on either end thereof. Themountings may for example take the form of hooks or eyes for engagingcomplementary hooks or eyes on the mounting bracket. The lengthadjusters typically take the form of screw adjusters.

In a fourth aspect of the invention, there is provided a radiant heateras hereinbefore defined, in combination with a reflective skirt formounting on said mounting bracket.

The reflective skirt may have downwardly divergent walls, or may havegenerally parallel walls, or a combination thereof. The reflective skirtadvantageously has a reflective surface of the same composition as thereflective surface of the housing.

The reflective skirt may be mounted on the brackets, and mayadditionally be provided with mounting points for securing directly tothe housing. For example, the reflector skirt may be bolted or riveteddirectly to the housing, as well as being secured to the mountingbrackets.

An advantage of the radiant heaters hereinbefore defined is that oncethe optimal reflector configuration has been selected for a givenlocation in a building, a heater having the desired configuration maysimply and easily be fabricated by attaching to the housing anappropriately configured set of brackets and an appropriately configuredreflector skirt. The present invention thus provides a means ofoptimising the heating of a building.

As is evident from the foregoing, the present invention is particularly(although not exclusively) concerned with radiant heaters, ie heaters inwhich the object or room to be heated is so heated by radiation emittedby the heater. These can be contrasted with convective heaters in whichair in the vicinity of a heating element is heated by conduction, andthen distributed to a region to be heated.

Since the radiation emitted by a hot body is related to the temperatureof that body by a power law, it follows that increased efficiency can beobtained from a radiant heater by running it such that the heatingelement is as hot as possible. Heaters of the type proposed in GB2145218comprise an elongate tube into which is directed an ignited combustiblemix. The burning of the mix heats the elongated tube, which then emitsradiation.

The present inventor has found that one limiting factor on theefficiency of such a heater is the formation of "hot spots" on thesurface of the heater, where the flame comes into direct contact withthe wall of the tube. If the combustible mix is adjusted to provide ahigher running temperature, the number and temperature of such hot spotsincreases, eventually leading to failure of the element.

In its fifth aspect, the present invention therefore provides a heatercomprising an elongate combustion chamber, one end of which is adaptedto be supplied with a combustible mix, the chamber having an inner linerwhich extends from that one end along the interior of the chamber andinto which the combustible mix is supplied, the liner having a smallercross-section than the chamber, and being perforated. Thus, the flamecan be retained within the liner but supplied with air from the regionbetween the liner and the inner wall chamber, which can enter the linervia the perforations.

Since the problem of hot spot formation is at its most severe at the endwhere the combustible mix is supplied, but is less so or negligible atthe distant end of the elongated combustion chamber, it is not necessaryfor the liner to extend along the whole length of the combustionchamber. Indeed, it is preferred that the liner is shorter than thechamber, to reduce cost and simplify construction.

In a preferred form of this fifth aspect of the invention, at the firstend of the combustion chamber, the liner is provided with a flaredportion which extends out of the combustion chamber and into which thecombustible mix is directed. Thus, the combustible mix is more easilydirected into the liner, and a positive gap can be left between theflared portion and the inlet to the combustion chamber to allow air intothe combustion chamber.

In this fifth aspect of the invention, since the flame is kept separatefrom the wall of the heating element, the flame temperature can beincreased resulting in increased efficiency.

A suitable form for the combustion chamber is an elongate tube, and theinner liner can then be a smaller tube within that.

According to a sixth aspect, the present invention provides a heatercomprising a combustion chamber having an inlet for a combustible mix,the combustible mix including a fuel component and an air component,wherein at least the air component is heated prior to mixing by beingdirected past the combustion chamber.

Thus, less heat is wasted in raising the inlet air to the temperature ofthe flame, and accordingly the flame can be run more efficiently.

A particularly suitable arrangement for directing the air component pastthe combustion chamber is to provide the combustion chamber in elongateform, the inlet being at a first end, the air component being directedalongside the length of the chamber. Thus, the inlet air is heated for arelatively long time and hence more efficiently, particularly if itflows from a distant end of the combustion chamber to the first end of acombustion chamber.

It is preferred that the elongate chamber comprises two interconnectedportions lying alongside each other, with the air component beingdirected along the elongate region between the two portions. In thisarrangement, inlet air will be heated from both sides, and hence moreefficiently. A suitable form for such an elongate chamber is a singletube bent to form a U-shape.

Where the combustion chamber is elongate, it is inevitable that thefirst end, at which the combustible mix is supplied, will be hotter thanthe distant end, where combustion products and flue gas are released.Thus, at the first end the radiative power of the heater is at itsgreatest, whilst at the distant end it is possible that a majority ofthe heat is dissipated by conduction to the surrounding air and hencecan be lost by convection of that air.

Thus, in its seventh aspect, the present invention provides a heatercomprising an elongate combustion chamber supplied at an inlet with acombustible mix including a fuel component and an air component, whereinthe air component consists substantially of air taken from the vicinityof the end of the elongated combustion chamber distant from the inlet.By distant is meant spaced along the length of the combustion chamber.Thus, in cases where the elongate chamber is a single tube bent to forma U-shape the distant end of the chamber may well be spatially adjacentthe inlet.

It is preferred that the air component of the combustible mix consistsessentially of air taken from the vicinity of the combustion chamberdistant from the inlet.

In many situations, a heater might be required to operate for shortperiods, interspersed with rest periods. Thus, it is desirable for theheater to reach its equilibrium running state swiftly. For example, if aheater requires five minutes to reach its running state, but is onlyrequired to run for ten minutes at a time, then the heater will only berunning at peak efficiency for 50% of the time. Furthermore, it has beenappreciated by the inventor that the ratio of fuel and air components ofthe combustible mix will change as the temperature of the two componentschange. This arises simply because as the temperature increases, thevolume of a fixed mass of gas will increase and hence to maintainstoichiometric conditions, a correspondingly greater volume of air willbe required. Thus, if at or immediately after start-up the fuel-airmixture is set at that found to be suitable at the higher runningtemperature, then the reduced burning efficiency at lower temperatureswill mean that the heater will take longer than necessary to reach itsrunning temperature.

Therefore, the present invention provides in its eighth aspect a heatercomprising a combustion chamber, a flue, and a suction means for drawingair out of the flue from the combustion chamber, the volume rate of airdrawn from the suction means being dependant upon the temperature of theair being drawn.

A bimetallic element is suitable for detecting the temperature. Whilstthis temperature detection could be done actively, by sensing theposition of the bimetallic element or the return from another suitabletemperature sensing element, and adjusting the suction meansaccordingly, it is preferred in this eighth aspect of the presentinvention that the bimetallic element itself partly covers the flueoutlet and is arranged such that it progressively further uncovers thatoutlet as the temperature of the element increases. Hence, adjustment ispassive, ie. automatic and requires no adjustment or complex activeinteraction.

Greater effectiveness can be achieved by this arrangement if it iscombined with the provision of further flow restrictions in thecombustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated, by way of example, by referenceto the accompanying drawings in which:

FIG. 1 is a plan view from above of a radiant heater according to oneembodiment of the invention;

FIG. 2 is a plan view from below of the embodiment of FIG. 1;

FIG. 3 is a sectional elevation along line I--I in FIG. 2;

FIG. 4 is a sectional elevation along line II--II in FIG. 2;

FIGS. 5 to 8 illustrate the embodiment of FIGS. 1 to 4, but with varyingreflective skirt configurations;

FIG. 9 is a sectional elevation of the bracket shown in FIG. 3;

FIG. 10 is a view of a heater according to a second embodiment of thepresent invention from the underside;

FIG. 11 is a section on III--III of FIG. 10;

FIG. 12 is a partial cross-section on IV--IV of FIG. 11, and as such isan enlarged, partially sectional view of the inlet portion of FIG. 10;and

FIG. 13 is a diagrammatic illustration of the air vent of FIG. 12.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

Referring now to FIGS. 1 to 4, it can be seen that in one embodiment,the radiant heater comprises a housing generally designated 1 having anouter wall 2 formed of mild steel and formed so as to have a generallyhorizontal region 2a and downwardly divergent portions 2b and 2c.Secured to the outer wall 2 by means of riveted joints at location 2d isan inner wall 3, formed of bent aluminium sheet, the downwardly facingsurface of which has been anodised, and preferably provided with goldcolour. Inner wall 3 is shaped so as to define two downwardly opensub-channels 5 and 6, each of the sub-channels having an upperreflective surface 5a, 6a, and downwardly divergent lateral reflectivesurfaces 5b, 5c, 6b, 6c. Surfaces 5c and 6c, together with a linkinglower wall 7 form a central barrier portion 8, the function of whichwill become apparent from the following description. At spaced (e.g. onemeter) intervals along the housing, brackets 9 are secured to thehousing. Bracket 9 is illustrated in FIG. 9, where it can be seen thatthe bracket has a generally horizontal cross-bar portion 101 formed ofbox section steel and, secured thereto, by means of bolts 102, agenerally upright member 103 at the upper ends of which are securedattachment brackets 104 of channel section. At the midpoint of thecross-bar portion 101, is secured, by welding, a short transversallymounted piece of steel box section 105 from the upper corners of whichextend divergent arms 106, which in use are arranged to embrace, but arenot fixedly attached to, the central barrier portion 8 of the housing.The bracket is secured to the housing by means of mounting fixtures 104which fit over the lower edges of the housing and are secured in placethereon by means of bolts 10.

The brackets 9 are provided with inwardly facing pairs of hook elements107 which engage the retaining rings 11 on the respective ends oftube-supporting cables 12.

Tube-supporting cables 12 are typically formed from a flexible hightemperature resistant metallic material such as steel, and are providedwith screw adjusters 13 formed from a non ferrous metal such as brasswhich allow the cables 12 to be shortened or lengthened. Burner tubes 15and 16 rest loosely on the cables 12 and, as will be appreciated, theheight of the tube within the housing may be varied by shortening orlengthening the supporting cables 12.

The burner tubes 15 and 16 extend along the channel from one end of thehousing to the other, tube 15 being connected at one end 17 with a gasburner (not shown) which heats the interior of the tube. Combustiongases are drawn along the tube from the burner 17 via a U-bend (notshown) at location 19 and into the return tube 16 by means of anextraction fan (not shown) mounted at end 18.

The tubes 15 and 16 are formed from steel, and may be surface treated tomaximise their radiative efficiency. In use, the tube 15 is heated bymeans of the gas burner and then functions as a radiator heatingelement, with radiation from the surface of the tube being reflected byreflective surfaces 5a, 5b and 5c in a downwards direction.

Tube 16 also gives out radiation, but to a lesser extent since the tubeis somewhat cooler than tube 15.

In order to prevent conductive and convective losses through the uppersurface of the housing, a layer of insulation 14 is disposed between theinner and outer walls. The layer of insulation 14 fills the spacebetween the inner 3 and outer 2 walls except at location 14a, where thesurface 14a of the insulating material, together with walls 5c and 6c ofthe central barrier portion 7 define a hollow channel running along thelength of the housing.

The thermal insulating material is selected so as to be resistant to theoperating temperatures of the heater, and for example may be selected soas to resist temperatures of 600° C. and above.

As illustrated in FIG. 4, the housing has secured to the lower edgesthereof a reflective skirt comprising side panels 19 having inwardlyfacing anodised aluminium reflective surfaces 19a. Panels 19 are securedto the housing by means of rivets 20 and are also mounted on, and heldrigidly in place by, brackets 9. The reflector skirt 19 serves to focusand reduce the angle of spread of radiation from tubes 15 and 16.

The reflective skirt 19 may be replaced by reflector skirt 21, 22, 23 or26 as illustrated in any one of FIGS. 5 to 8 in order to vary the angleof spread of the radiation from the heater tubes. For example, when itis necessary to mount the heaters at a higher point within a building,e.g. as a result of the roof or ceiling support structure or otheravailable supporting structures being much higher above the ground, alonger reflective skirt as illustrated in FIG. 6 may be employed toreduce the spreading of the radiation thereby to provide the desiredradiative flux density at ground level. Conversely, where it isnecessary to mount the heaters at a lower point in a building, thereflective skirt shown in FIG. 4 may be replaced by the shorterreflective skirt shown in FIG. 5.

In FIGS. 5 and 6, the reflective skirts are shown as having generallyparallel downwardly extending walls, but they may also, for example, beinclined, as illustrated in FIGS. 7 and 8, where the upper parts 24 and27 respectively of the reflective skirts are divergent and follow thelines of the housing, and the lower parts 25, 28 of the reflectiveskirts 23, 26 respectively are substantially parallel.

When a heating system for a building incorporating the radiant heatersof the invention the building floor area A is first measured and thedesired temperature rise ΔT above ambient is selected. From the floorarea A and ΔT, the required radiant flux density Q at floor level isthen determined. Taking into account the height at which the heaters areto be suspended within the building, and taking into account also theshape of the floor area, an array of heaters is then chosen, each heaterhaving a reflective skirt of the appropriate configuration to providethe desired radiant flux density at its given location in the building.As will be appreciated, the configuration of a reflective skirt for aheater in a corridor, alcove or bay would be different from theconfiguration of the reflective skirts on heaters in the main hall of abuilding.

An advantage of the embodiments of the present invention specificallyset forth above is that they provide a basic radiant heater which canreadily be adapted to provide the desired radiant flux density at agiven location in a building by selecting an appropriately shapedreflector skirt. The radiant heaters according to this embodiment of theinvention thus offer significant advantages over presently availableradiant heaters which tend to be of fixed configuration and do not havethe facility for modification in the manner illustrated above.

A further aspect of the present invention is exemplified by the heaterillustrated in FIG. 10. The heater 110 comprises a substantiallyU-shaped heater element 112 comprising a pair of linked generallyparallel heater tubes 112a and 112b. Between the tubes 112a and 112b isa flow passage 114 having a closed distant end 116 lying in the base ofthe U defined by the heater tube 112. Louvres 118 are provided on theside of the flow passage 114 facing tube 112b, along roughly one-thirdof the length of the flow passage 114 nearest its distant end 116. Theends of the tubes 112a, 112b and flow passage 114 are enclosed in acompartment 120. The interior of the compartment 120 is shown in moredetail in FIG. 12, described later.

FIG. 11 shows the heater in cross-section. It can be seen that the outercasing 122 comprises a generally hollow section filled with aninsulating material 124. The casing 122 has side walls 122a, 122b.Suspended from the casing 122 is a hollow truncated V-section, whichforms the flow passage 114 and which runs along the length of the casing122. Thus, the casing 122, side walls 122a and 122b, and flow passage114 between them define two elongate regions. Within these elongateregions are suspended the heater tubes 112a and 112b respectively. Thesuspension is achieved by a suspension means, not shown in FIG. 11. Thiscan be as shown in the embodiments of FIGS. 1 to 9.

FIG. 11 also shows that tube 112a has an inner liner tube 126 which liesgenerally concentrically within tube 112a and is perforated byperforations 128.

Referring to FIG. 12, this shows the region about the enclosure 120 intowhich project the heater tubes 112a and 112b. Heater tube 112a can beseen to contain the inner liner tube 126 along part of its length,although both the inner liner tube 126 and heater tube 112a arecoterminus at an open end within the enclosure 120. Inner liner tube 126is, as previously mentioned, perforated by perforations 128. At the openend, the inner liner 126 is provided with a flared inlet 130. Facing theinlet 130 is a burner 132 supplied with fuel. Burner 132 is a standarditem.

The heater tube 112b has an open end extending into the enclosure 120,where it is connected to a suction fan 134 which is arranged to extractgas from the heater tube 112b and vent it to atmosphere through a ventnot shown in FIG. 12.

The interior of the enclosure 120 is partitioned to prevent gas flowbetween the free ends of the heater tubes 112a and 112b. The flowpassage 114 communicates with the region into which tube 112a projects.

FIG. 13 shows the vent 136 of the suction fan 134. The vent 136 has anopening 138 which is partially covered by a bimetallic element 140. Whenair being expelled from the vent 136 through the opening 138 is cool,the bimetallic strip 140 is flat and is in position (i), almostcompletely covering the opening 138. Thus, the flow out of the vent 136is restricted. As the temperature of gas flowing out of the opening 138increases, the bimetallic element 140 bends away from the opening 138through position (ii) and progressively into position (iii), thusreducing the restriction on flow and allowing more gas to pass.

It can be seen that in general, only part of the opening 138 isuncovered at any one time, but in the generally spiral outlet employedin this embodiment, this does not matter because escaping gas generallyfollows the route shown by arrow A. Thus, a greater proportion ofescaping gas passes through the outer third of the outlet 138 and hencein its fully withdrawn position (iii) the bimetallic element 140 allowsa sufficient volume of gas to pass.

The operation of the heater 110 of the present invention is generally asfollows. The suction fan 134 draws air along the tube 112b, around theU-bend in the heater tube 112, and hence along the tube 112a. Thus,there is a negative pressure in the region of the burner 132. For thisreason, air is drawn along the flow passage 114, being supplied to thepassage via louvres 118. Since the louvres face the heater tube 112b,air will be drawn from the vicinity of that tube. Once the heater isrunning, air will remain in the elongate space surrounding the tube 112bthrough convection, and therefore can be expected to flow into thelouvres 118 from along the entire length of the tube 112b.

Once it reaches the burner 132, air mixes with fuel and is ignited whenit passes into inlet 130. Inlet 130 ensures that all flames pass intothe inner liner 126, where they are fed with secondary air flowing fromthe space between the inner liner 126 and the burner tube 112a viaperforations 128. Hence, inner liner 126 protects the burner tube 112afrom the extreme temperature of the flames in the vicinity of the burner132. However, since the temperature of the flame will decline along thelength of the burner tube 112, the inner liner 126 is not required alongthe entire length and hence is shorter than the burner tube 112.

Inevitably, the tube 112a will be hotter than the tube 112b, and thesetwo tubes will themselves have a graduated temperature therealong.However, the provision of the tubes in a U-formation means that, alongthe length of the heater, the average temperature of the two tubesremains substantially constant. Thus, the total radiative output of theheater is substantially constant along its length. In addition, the endof the tube 112b nearest the suction fan 134 will be at a such lowtemperature that its radiative efficiency will be very low compared tothe equivalent portion of the burner tube 112a. However, this is not aproblem in the present invention since the air around tube 112b, whichwould normally escape through convection without contributing to theradiative power of the heater, is instead drawn alongside tube 112b,through louvres 118, and used as pre-heated combustion air.

The heater 110 of the present invention is able to reach its operatingtemperature more quickly, due to the temperature-dependent restrictionon the outlet 136, described above. Thus, when fully cold, the heateroperates in a fuel-rich state in which there is little air (by volume)flowing along heater tubes 112. Thus, the working temperature is reachedmore swiftly. However, once that working temperature is reached, theflow restriction on the outlet 136 is substantially removed. This effectcan be enhanced, if desired, by providing flow restrictions such asbaffles within the tube 112b.

It will readily be apparent that numerous modifications and alterationsmay be made to the radiant heaters illustrated in the drawings anddescribed above, without departing from the principles underlying thepresent invention, and all such modifications and alterations areintended to be embraced by this Application.

What is claimed is:
 1. A radiant heater, comprisinga radiative heatingelement, a housing, the underside of which is recessed to define a pairof channels arranged side by side and separated by a central barrierwhich tapers in a downward direction, the radiative heating elementincluding a pair of radiant heater tubes each being disposed beneath thehousing such that an upper half of said heater tubes is disposed whollywithin the channel, and at least a portion of a lower half of saidheater tubes protrudes downwardly from the channel, each said channelhaving a heat reflective surface for reflecting heat radiation from theradiant heater tube in a downwards direction, and wherein the housingincludes means for enabling the attachment thereto of a reflective skirtfor focusing the radiation emitted from the radiative heating element,wherein the housing is provided with a plurality of mounting bracketsfor attachment beneath the housing at spaced intervals thereon, themounting brackets having first mounting means for supporting the tubebeneath the housing, and second mounting means for the attachmentthereto of the reflective skirt.
 2. A radiant heater according to claim1, wherein the radiant heater tube is heated by a gas burner or anelectrically heated heating element.
 3. A radiant heater according toclaim 1, wherein the radiant heater tube comprisesa burner communicatingwith one end of the tube, and extraction means communicating with theother end of the tube for extracting combustion gases from the tube. 4.A radiant heater according to claim 1, wherein the pair of radiantheater tubes constitute two limbs of a U-tube burner, the burner beingarranged to communicate with one end of the U-tube, and the extractionmeans being arranged to communicate with the other end.
 5. A radiantheater according to claim 1, wherein the radiant outputs of the radiantheater tubes are matched such that the radiant heater has asubstantially uniform total radiant output along its length.
 6. Aradiant heater according to claim 1, wherein the central barrier extendsdownwardly such that a lower edge of said barrier is aligned with loweredges of the walls of the channels.
 7. A radiant heater according toclaim 1, wherein the central barrier has opposing walls which have anangle of inclination or a curvature which mirrors an angle ofinclination or curvature of an opposing wall of the channel.
 8. Aradiant heater according to claim 1, wherein each said channel isadapted to function as a parabolic reflector.
 9. A radiant heateraccording to claim 1, wherein each said channel has a generally planarupper surface, and generally planar side surfaces diverging downwardlyfrom edges thereof.
 10. A radiant heater according to claim 1, whereinthe mounting brackets are attached to the housing by means providing aremovable or fixed connection with the housing.
 11. A radiant heateraccording to claim 1, wherein the housing is thermally insulated tolimit heat loss through conduction and convection via upper and sidesurfaces of the housing.
 12. A radiant heater according to claim 1,wherein the heat reflective surfaces are treated to reduce surfaceporosity and unevenness to improve reflectance.
 13. A radiant heateraccording to claim 12, wherein the heat reflective surfaces are formedof an anodized aluminum.
 14. A radiant heater according to claim 13,wherein the heat reflective surfaces are formed of a colored anodizedaluminum.
 15. A radiant heater according to claim 1, further comprisingmeans for adjusting a height of one of the tubes within the housing. 16.A radiant heater according to claim 15, wherein the means for adjustingthe height of the tube within the housing comprises one or moreadjustable length cables extending between opposing mounting points onthe mounting bracket, said cables and tubes being supported on saidmounting bracket, the cables being adjustable such that shortening thelength of the cable raised the height of the tube, lengthening the cablelowers the height of the tube.
 17. A radiant heater according to claim16, wherein the adjustable length cable comprises a flexible stainlesssteel cable having non-ferrous mountings and length adjusters on eitherend thereof.
 18. A radiant heater as defined in claim 1, in combinationwith a reflective skirt for mounting on said mounting bracket.
 19. Aradiant heater according to claim 18, wherein the reflective skirtcomprises one or more downwardly divergent walls, or generally parallelwalls, or a combination thereof.
 20. A radiant heater according to claim19, wherein the reflective skirt comprises a reflective surface of thesame composition as the reflective surface of the housing.
 21. A radiantheater according to claim 19, wherein the reflective skirt comprisesmounting points for securing said skirt directly to the housing.
 22. Aradiant heater according to claim 21, wherein the reflector skirt isadapted to be or riveted directly to the housing, and secured to themounting brackets.
 23. A process for fitting a heating system in abuilding, the heating system comprising overhead radiant heaters withskirt portions for directing the radiated heat towards a floor of thebuilding, the process comprising the steps ofmeasuring the buildingfloor area A, determining the desired temperature rise ΔT above ambient,from A and ΔT, determining the required floor radiant flux density Q,from Q and the performance of a heater, determining the number N ofheaters, from A and N, determining a desired floor radiant flux patternfor an individual heater, from the floor radiant flux pattern, selectinga skirt having a configuration to achieve that pattern, attaching theselected skirt to a heater and installing the heater and skirt in thebuilding.